The invention relates to a device for projecting the image of a reticle in an optical system. The line of sight of the optical system is represented by the reticle.
The reticle-projecting device comprises a collimator situated at the edge of the pupil of an objective of the optical system. The objective has an optical axis which coincides with the line of sight.
The invention is employed mainly, though not exclusively, in thermal imagers used for aiming guns or missiles. In such applications, the line of sight should be brought into line with the weapon system. The firing precision depends on the accuracy of this alignment, which should be maintained under the conditions in a military environment.
There are several methods of forming a reticle. One of these methods is to arrange a collimator in front of the thermal camera. The collimator projects an image of a reticle into the camera, and the image is superimposed on the image of the scene. In this case the stability of the line of sight depends only on the collimator. This is important because it is much easier to ensure a stable arrangement of a collimator of small dimensions than that of a thermal camera, which is a complex optical apparatus comprising bulky optical systems.
The collimator must be situated in the center of the pupil of the thermal camera. For any other arrangement of the collimator, for example at the edge of the pupil, a focussing error will lead to an angular displacement of the line of sight.
This arrangement of the collimator in the center of the pupil of the camera is not always possible because of the available space or the mechanical connection. If the camera is equipped with a zoom or multifocus objective, arrangement of the collimator at the center of the pupil will block the entire scene when the diameter of the pupil decreases for short focal lengths.